Internal combustion engines

ABSTRACT

An exhaust gas recirculation system for an internal combustion engine comprises a metering orifice in the exhaust system of the engine opening into a recirculation conduit which leads to the inlet manifold of the engine, a pressure-control valve assembly which controls communication between a location in said conduit immediately downstream of the metering orifice and the inlet manifold in response to changes in pressure at said location such as to maintain said location at substantially atmospheric pressure. The rate of recirculation through said conduit is therefore a substantially constant percentage of the mass airflow rate of the engine. A pressure sensing valve assembly is responsive to a pressure in the engine supply system which indicates certain operating conditions of the engine and automatically closes the valve of the pressure-control valve assembly to prevent exhaust gas circulation under said operation conditions.

United States Patent [1 1 Swatman Apr. 9, 1974 INTERNAL COMBUSTION ENGINES (22] Filed: Mar. 30, 1972 [2!] Appl. No.: 239,670

[76) inventor:

152] U.S. Cl l23/ll9 A [51 Int. Cl. .r FOZm 25/06 15%| Field of Search l23/l I9 A". 60/278, 279

{56] References Cited UNITED STATES PATENTS 3,368,345 3/1968 Walker l23ll l9 B X 3.49 I ,736 1/2970 Walker l23/l I) A 3.662,"!22 5/!972 Sarto i231! 19 A R22.994 4/!948 Bicknellmt 1231i H? A 2,722.92? ll/l955 Cornelius llll 123/! I9 A 3,64l989 2/l972 Hill l23/l 19 A 3,643,640 2/l972 Kraus et al l23/i i9 A Primary Examiner-Al Lawrence Smith Assistant Examiner-W. H. Rutledge, Jr. Attorney, Agent, or Firm-Brisebois & Kruger I 5 7 ABSTRACT An exhaust gas recirculation system for an internal combustion engine comprises a metering orifice in the exhaust system of the engine opening into a recirculation conduit which leads to the inlet manifold of the engine, a pressure-control valve assembly which controls communication between a location in said contluit immediately downstream of the metering orifice and the inlet manifold in response to changes in pressure at said location such as to maintain said location at substantially atmospheric pressure. The rate of recirculation through said conduit is therefore a substantiaily constant percentage of the mass air-flow rate of the engine. A pressure sensing vaive assembly is responsive to a pressure in the engine supply system which indicates certain operating conditions of the engine and automatically closes the valve of the pressure-control valve assembly to prevent exhaust gas circulation under said operation conditions.

9 Claims, 3 Drawing Figures iMENTEB APR 9 I974 SHEEI 1 Bf 3 m mm rl NM, WV 0 1 w Q III A 41. "i I'd 9 PMENIEDAPR 91914 3.802 402 SHEET 3 or 3 INTERNAL COMBUSTION ENGINES This invention is concerned with means for reducing atmospheric pollution by exhaust emissions from internal combustion engines.

Exhaust gas recirculation is well established as a means of reducing the emission of oxides of nitrogen (commonly represented as N by the exhaust system of an internal combustion engine. But exhaust gas recirculation systems employed hitherto leave much to be desired, as regards reducing N0, emissions, simply because the control exercised on the amount of exhaust gas recirculated is too imprecise, and also varies with the operating conditions of the engine.

The invention, which is applicable both to an engine with carburetter equipment and to one with a petrolinjection system, is aimed at providing an exhaust gas recirculation system which: (I controls the amount of exhaust gas recirculated so that this amount is a substantially constant proportion (e.g. from about 12 to about 17 percent) of the mass air-flow rate of the engine; and (2) preferably is controlled automatically to effect recirculation only when the operating conditions of the engine are such as to require it. With regard to 2, recirculation is not required in any of the following operating conditions: idling, overrunning, full load, or when the engine is running at a temperature which is below a predetermined level. in particular, recirculation is not required at cold-starting, nor during the initial phase of warming-up, but is required when the vehicle is being driven with the engine at a temperature of 60F. or more.

There are several different ways of determining the mass air-flow rate of any given internal combustion engine, but I consider that the most practical method in this context for application to motor vehicles is to utilize the back-pressure of the exhaust gas as a criterion. The mass of exhaust gas produced by the engine per unit time is directly proportional to the mass air-flow rate of the engine.

The principle underlying the invention can readily be appreciated by notionally regarding the exhaust system of the engine as comprising two simple orifices, primary and secondary respectively; each discharging exhaust gas into the atmosphere. That is to say, the respective effluxes occur into zones of identically the same pressure. Consequently, the mass flow rate through each orifice will be proportional to the efi'ective cross-sectional area of the individual orifice. Therefore, if the effective cross-sectional area of the secondary orifice were about percent of that of the primary orifice (which, as already postulated, discharges into atmospheric pressure), and the efflux from the secondary orifice was discharged into a zone of atmospheric pressure before entering the induction system of the engine, then the mass rate of exhaust gas recirculation would be substantially 15 percent of the mass air-flow rate of the engine, irrespective of the temperature of the exhaust gas.

It will be apparent, from the foregoing, that, having established experimentally the correct percentage of the exhaust gas mass flow rate that should be recircu lated for a given engine installation, it merely remains to provide means effective automatically either to engender recirculation or to prevent it (as the case may be), depending upon the operating conditions of the engine at any given time.

According to this invention an internal combustion engine is equipped with an exhaust gas recirculation system in which: a metering orifice in parallel with the exhaust system governs the rate of recirculation as a substantially constant percentage of the mass air-flow rate of the engine; and a pressure-control valve assembly maintains substantially atmospheric pressure at a location immediately downstream of the metering orilice and also controls communication between that location and the inlet manifold. A subsidiary feature of the invention is that a pressure-sensing valve assembly, responsive to the operating conditions of the engine, automatically signals the pressure-control valve assembly to permit exhaust gas recirculation only when this is necessary.

More specifically, an internal combustion engine conforming to the invention and having means for controlling the rate of mass air flow into its induction system, is equipped with an exhaust gas recirculation system which effects automatic control of the recirculation at a substantially constant proportion of the mass air-flow rate only when the operating conditions of the engine are such as to require it, and which comprises: a metering orifice in parallel with the exhaust system and serving to govern the rate of recirculation as a substantially constant percentage of the mass air-flow rate of the engine; a recirculation conduit receiving the efflux of the metering orifice and leading to the inlet manifold from a location immediately downstream of that orifice; a pressure-control valve assembly that includes an exhaust-admission valve which controls communication between the recirculation conduit and the inlet manifold, and which, by its movements, maintains substantially atmospheric pressure in the recirculation conduit immediately downstream of the metering orifice whenever exhaust gas recirculation is required; and a pressure-sensing valve assembly which is responsive to the depression obtaining at the location of the usual vacuum ignition-advance drilling, and which is effec tive automatically to signal the pressure-control valve assembly to permit recirculation only when the operating conditions of the engine are such that exhaust gas recirculation is necessary.

In the preferred embodiment of the invention the pressure-control valve assembly comprises: a pressuresensitive member that senses atmospheric pressure at one side and, at its other side, the pressure immediately downstream of the exhaust gas metering orifice; and a pneumatic servo which operates the exhaust-admission valve in response to movement of the pressure-sensitive member.

Operation of the pressure-sensing valve assembly to signal the pressure-control valve assembly, in order to engender exhaust gas recirculation, is precluded by thermally-responsive means whenever the engine temperature is below about F.

Referring to the accompanying drawings:

FIG. 1 illustrates schematically the general arrangement of an exhaust gas recirculation system of an internal combustion engine incorporating the invention;

FIG. 2 illustrates schematically a system basically similar to that of FIG. 1, but in which some of the corresponding components are constructed and arranged differently; and

FIG. 3 is a sectional elevation of an alternative arrangement of a pressure-control valve assembly which is employed in each of the systems illustrated in FIGS. 1 and 2.

In the arrangement depicted in FIG. 1, an internal combustion engine 1 has a carburetter 2, an air cleaner 3, an induction pipe 4 (forming part of an inlet mani fold 5) and an exhaust pipe 6. The carburetter 2 (which in this instance is of the well-known controllable jet, automatic variable-choke type) has its mixture passage 7 fitted, as usual, with a throttle disc 8.

The exhaust gas recirculation system comprises: a metering orifice 9 in the exhaust pipe 6, a pressurecontrol valve assembly 10 and a pressure-sensing valve assembly 11. The valve assembly 10, which controls communication between the induction pipe 4 (downstream of the throtte disc 8) and a recirculation conduit 12 that leads from the metering orifice 9, serves to maintain substantially atmospheric pressure in the conduit 12 immediately downstream of the metering orifice 9 when recirculation is in progress. The pressuresensing valve assembly 11 determines when exhaust gas recirculation is to take place, by exercising a triggering effect upon the pressure-control valve assembly 10.

The pressure-control valve assembly 10 has a casing 13 containing three diaphragm-chambers 14, 15 and 16 which are formed by two diaphragms l7 and 18 arranged as shown. The intermediate chamber 15 is in permanent communication with atmosphere, by way of a duct 19 and an air cleaner 20. It should perhaps be pointed out that the separate air cleaner 20 is necessary because the supply of air to the chamber 15 must be at atmospheric pressure, and cannot be taken from downstream of the engine air cleaner 3.

Exhaust gas recirculation is, so to speak, turned on and off by an exhaust-admission valve 21. When this valve is moved off its seating 22, the efflux of the metering orifice 9 passes from the recirculation conduit 12 into the inlet manifold 5, so that exhaust gas recirculation then takes place.

The exhaust-admission valve 21 is of annular form and is rigidly fixed to one end of a hollow valve stem 23, which passes into the casing 13 through a flexible grommet-like gland 24 that affords a sliding pressure seal with the valve stem. The latter is attached to the diaphragm 17, but extends into the diaphragmchamber 15 so that an orifice 25 at the top of the hollow valve stem 23 is able to coact with a disc-like valve 26 fixed centrally to the diaphragm 18. The interior of the valve stem 23 is in permanent communication with the diaphragm-chamber 14 through an orifice 27; and a restrictor 28 is fitted in the hollow valve stem 23, either (as shown) adjoining the valve 21 or at a location between that valve and the orifice 27.

The exhaust-admission valve 21 moves under the influence of the diaphragm 17 which senses manifold depression (or a controlled part of it) applied to the chamber 14 by way of the restrictor 28, the hollow valve stem 23 and the orifice 27.

The diaphragm 17, which constitutes a pneumatic servo, is acted upon by a helical compression spring 29 which permits its required travel to open the valve 21 fully under a depression of approximately 2% inches Hg (which is not quite the value of the full-load inlet manifold depression of the engine). The effective area of the diaphragm 17 has to be such that this depression,

acting on it, overcomes the spring 29. It should perhaps also be pointed out that, when no exhaust gas recirculation is taking place, the spring 29 has to be strong enough to ensure that the valve 21 is held to its seating 22 against such manifold depression or exhaust gas pressure as may be tending to unseat this valve. That is to say, the spring 29 has to apply a given load for a given area of the valve 21.

A compression spring 30, trapped between the diaphragms 17 and 18, is just strong enough to support the weight of the diaphragm 18 when the control valve assembly 10 is disposed vertically, as shown.

The depression acting on the diaphragm 17 is a function of: the depression existing in the inlet manifold 5, the effective area of the restrictor 28 and the effective area of the orifice 25 opened to the atmospheric connection 19 by the valve 26 on the diaphragm 18.

Movement of the valve 26, which varies the area of the orifice 25, is controlled by movement of the diaphragm 18 when it senses the pressure immediately downstream of the metering orifice 9 via a pipe 3], connecting the diaphragm-chamber 16 to the pressuresensing valve assembly 11, and a pipe 32 which leads to that valve assembly; the pipe 32, which emerges from the recirculation conduit 12, having an orifice 33 located immediately downstream of the metering orifice 9.

When the pressure in the diaphragm chamber 16 is greater than atmospheric, the diaphragm 18 is deflected so that its valve 26 closes the orifice 25. Consequently, the available manifold depression now acts on the diaphragm 17 which moves against its spring 29 and causes the valve 21 to open, so that exhaust gas recirculation occurs.

When the pressure in the diaphragm-chamber 16 is less than atmospheric, the orifice 25 is opened and, therefore, more nearly atmospheric pressure obtains in the diaphragm-chamber 14. Under the action of its spring 29, the diaphragm now causes the valve 21 to close: the movement of this valve being automatically adjusted between the open and closed' positions so that a greater or lesser depression is created in the conduit 12 to maintain substantially atmospheric pressure downstream of the metering orifice 9, irrespective of the exhaust gas pressure acting on that orifice and irrespective of the inlet manifold depression acting on the valve seating 22.

The various operating conditions of the engine in which exhaust gas recirculation is not required have been specified above, and need not be repeated. As already indicated, the purpose of the pressure-sensing valve assembly 11 is to signal the pressure-control valve assembly 10 either to effect exhaust gas recirculation or to preclude this, depending upon the operating conditions of the engine at any time.

The pressure-sensing valve assembly 1] has a casing 34 containing two diaphragm-chambers 35 and 36 which are formed by a diaphragm 37. This is loaded by a helical compression spring 38, and has an adjustable stop 39 limiting its outward travel. From a throttle-edge drilling 40 of the carburetter 2, a pipe 41 is connected to the diaphragm-chamber 35. The drilling 40 is similar in function to the so-called vacuum ignition-advance drilling employed in modern carburetters. A duct 42 connects the diaphragm-chamber 36 with a valve housing 43 which has alternative annular seatings 44 and 45 for a disc-like valve 46. The latter is mounted on one end of a stem 47 that passes through the duct 42, the other end of this stem being articulated to the thermally-deflectable limb of a substantially U-shaped bimetallic element 48 of which the other limb is rigidly fixed to the central part of the diaphragm 37. Thus, the valve 46 is movable both by the diaphragm 37 and by thermal deflection of the interposed element 48. in the manner shown, the pipes 31 and 32 (referred to earlier) respectively connect the valve housing 43 to the diaphragm-chamber 16 of the pressure-control valve assembly 10, and to the orifice 33 located immediately downstream of the exhaust gas metering orifice 9. And a pipe 49 interconnects the duct 42 and the region 7 lying between the variable choke (not shown) and the throttle disc 8 of the carburettor 2. As this region 7 is one in which a substantially constant depression exists whenever a carburetter of the above-described type here involved is operating, it is commonly designated simply the constant depression region.

The load imposed by the spring 38 in association with the effective area of the diaphragm 37 is such that the latter makes a full excursion to the stop 39 under a depression of approximately 1 psi in the chamber 35; the other diaphragm-chamber 36 being in communication, via the pipe 49, with the constant depression region 7 of the carburetter 2.

When the throttle 8 of the carburetter 2 is virtually shut, as at idling, only the depression existing in the constant depression region 7 acts in the diaphragmchamber 35, and the diaphragm 37 does not move. This diaphragm is now in a position such that the valve 46 is on its seating 45, sealing the end of the pipe 32; and the diaphragm-chamber 16 of the pressure-control valve assembly is connected via the pipes 31 and 49 to the constant depression region 7 of the carburettor. As a result, the diaphragm 18 of the valve assembly 18 lifts the valve 26 off the top of the hollow valve stem 23, so that the diaphragm-chamber 14 is at atmospheric pressure (by way of the orifices 25 and 27) and the servo-diaphragm 17 keeps the valve 21 closed, thus precluding exhaust gas recirculation.

When the throttle disc 8 is opened, the chamber 35 of the valve assembly 11 is subjected to an increasing degree of inlet manifold depression which, acting on the diaphragm 37, causes the valve 46 to move onto its seating 44 so that it closes off the connection to the constant depression region 7 (by way of the pipe 49). At the same time, the valve 46 has opened communication between the pipes 31 and 32. Consequently, the diaphragm-chamber 16 of the valve assembly 10 is now pressurized from the exhaust gas orifice 33, and recirculation ensues.

When the throttle disc 8 is opened so widely that the engine is on full load, the depression acting in the chamber 35 of the valve assembly I] is insufficient to hold the diaphragm 37 against the thrust of its spring 38, and consequently the valve 46 is moved off its seating 44, so that the pipes 31 and 49 effect communication between the constant depression region 7 of the carburetter 2 and the diaphragm-chamber 16 of the valve assembly 10. Then, in the way already described, the servo-diaphragm l7 closes the valve 2] which turns off the recirculation.

Under cold conditions the thermally-deflectable limb of the bi-metallic element 48 assumes the dotted line position, with the result that when the engine temperature is below approximately 60F. the diaphragm 37 comes against the stop 39 without moving the valve 46, and recirculation does not occur.

In practice, a maximum depression occurs in the constant depression region 7 of the carburetter when the engine is running at its maximum speed on full load, and when the air cleaner 3 is so clogged with dirt that the restriction which it imposes on the air flow is also at a maximum. The strength of the spring 38 is selected so that movement of the diaphragm 37 does not occur under this worst condition.

In overrun conditions the throttle disc 8 is shut, inadequate depremion obtains at the throttle-edge drilling 40 (and hence in the diaphragm-chamber 35) to move the valve 46 onto its seating 44, and the diaphragmchamber 16 of the pressure-control valve assembly 10 is put into communication (by the pipes 31 and 49) with the constant depression region 7 of the carburetter; with the result that exhaust gas recirculation is preeluded by closure of the valve 21 in the manner previously explained.

Referring now to FIG. 2, those components which are constructed and arranged in the same manner as in FIG. 1 are identified by the same reference numerals. The chief differences between the two systems lie in the exhaust gas metering orifice and in the pressuresensing valve assembly.

in the case of the system depicted in H0. 2, the exhaust pipe 6 has a stub-like branch 50 in which there is a metering orifice 9A penetrated by a reciprocatable plug-like valve 51. The metering orifice 9A, which is established by an annular seating 52 for the valve SI, is kept free from incrustation by particulate deposits because, each time it moves, the valve 51 exerts (on the surrounding edge of the orifice 9A) a rubbing action which ensures maintenance of the original effective area of the metering orifice.

The exhaust valve 51, which functions either fully open or fully closed, is carried by a stem 53 which passes slidably through a chamber 54 and into a casing 55. The latter contains a diaphragm 56 loaded by a he lical compression spring 57 which is located in a diaphragm-chamber 58. The valve stem 53, which is at tached centrally to the diaphragm 56, is fitted with a stop device 59 which, by contacting the top of the chamber 54, limits the opening movement of the exhaust valve 51 and also seals that chamber. When the exhaust valve 51 is open, the efflux of the metering orifice 9A passes from the chamber 54 to the inlet manifold S by way of a recirculation conduit 12A and the valve 21.

When the diaphragm-chamber 58 is subjected to the inlet manifold depression (in a manner which will be described later), the diaphragm 56 normally opens the exhaust valve 51. But the relationship between the effective area of the diaphragm 56 and the load imposed by the spring 57 is such as to preclude opening of the exhaust valve 51 when the diaphragm-chamber 58 is subjected to the maximum depression obtaining in the inlet manifold 5 at full-load operation of the engine.

The diaphragm 18 of the pressure-control valve assembly l0 senses the pressure immediately downstream of the exhaust gas metering orifice 9A through a pipe 60, which interconnects the diaphragm-chamber l6 and the exhaust gas chamber 54. A pipe 61 connects the diaphragm-chamber 58 to a pressure-sensing valve assembly 11A that includes a disc-like valve 46A hav ing alternative annular seatings 44A and 45A in a valve block 62.

The valve 46A is mounted on a stem 63 which passes slidably (and in gas-tight manner) through one end of the valve block 62 and into a casing 64, of which the adjoining zone is open to atmospheric pressure. The casing 64 contains a diaphragm 65 loaded by a helical compression spring 66 which is located in a diaphragmchamber 67. The valve stem 63, which is attached centrally to the diaphragm 65, is fitted with a stop device 68 that coacts with the thermally-deflectable limb of a substantially U-shaped bi-metallic element 69, the other limb of which is rigidly fixed at 70. The arrangement is such that whenever the engine temperature is below about 60F. the free end of the bi-metallic ele ment 69 prevents movement of the stop device 68 to the right, as shown, so that the valve 46A cannot be moved off its seating 45A. But when the engine temperature exceeds about 60F. the thermally-deflectable limb of the element 69 is in the position indicated by the dotted line, and no longer prevents the valve 46A from being moved off its seating 45A.

The valve block 62 has a duct 71 which is permanently open to atmosphere, and which is so located with respect to the valve seatings 44A and 45A that when the valve 46A (as depicted) is off its seating 44A the pipe 61 communicates atmospheric pressure to the diaphragm-chamber 58. The exhaust valve 51 is then kept closed by the spring 57.

When the operating conditions are such as to require exhaust gas recirculation, the increased depression communicated to the diaphragm-chamber 67 of the pressure-sensing valve assembly 11A causes the diaphragm 56 to move the valve 46A off the seating 45A and onto the seating 44A. In consequence, by way of a pipe 72 connecting the inlet manifold to the valve block 62 at a location open to the bore of the valve seating 45A, the pipe 61 now communicates the inlet manifold depression to the diaphragm-chamber 58. The diaphragm S6 thereupon moves against its spring 57 and opens the exhaust valve 51. As a result, the pressure immediately downstream of the exhaust gas metering orifice 9A is transmitted from the chamber 54, by way of the pipe 60, to the diaphragm-chamber 16 of the pressure-control valve assembly 10. The latter now admits exhaust gas into the inlet manifold 5 (in the manner already described for FIG. 1 thus maintaining substantially atmospheric pressure downstream of the metering orifice 9A.

Referring now to FIG. 3, this depicts an alternative arrangement of part of the pressure-control valve assembly 10 of FIGS. 1 and 2; the purpose of this alternative arrangement being to obviate, as far as possible, the likelihood of the valve stem 23 (FIGS. 1 and 2) eventually sticking in its gland 24 due to accretion of particulate carbon deposited on the valve stem 23 by the exhaust gas to which it is exposed. Accordingly, in the alternative arrangement (FIG. 3), the previous valve stem 23 is replaced by a similar hollow valve stem 23A which is remote from the exhaust-admission valve, and which is removed from the influence of the exhaust gas whenever this is delivered to the inlet manifold 5 from the recirculation duct 12 or 12A. The previous annular exhaust-admission valve 21 is replaced by a disc-valve 21A which has a seating 22A at the outlet of the recirculation duct 12 or 12A, and which is rigidly connected to the remote valve stem 23A by a hook-like yoke 73. This yoke is a T-section casting having at its ends accurately aligned holes 74 and 75 into which the valve 21A and the valve stem 23A are respectively pressed.

The yoke 73 operates within a chamber 76, to which access is afforded by a removable sealing plug 77. The base of the assembly shown in FIG. 3 is bolted directly to the inlet manifold (FIGS. 1 and 2), downstream of the throttle disc 8, with the chamber 76 lying nearest to the throttle disc 8. As a result, the remote valve stem 23A becomes exposed only to fuel vapour in the chamber 76, and does not suffer contamination by exhaust gas when the exhaust-admission valve 21A is open, because the chamber 76 leads from the inlet manifold 5 (FIGS. 1 and 2) at a location upstream of that at which the recirculated exhaust gas enteres this manifold.

I claim:

1. An exhaust gas recirculation system for an internal combustion engine equipped with an inlet manifold and an exhaust system, said recirculation system comprising a metering orifice in said exhaust system opening into a recirculation conduit which leads to said inlet manifold, and

a pressure-control valve assembly comprising a pressure-sensitive member subjected to atmospheric pressure on one side and to the pressure in said conduit at a location immediately downstream of the exhaust gas metering orifice on its other side, an exhaust-admission valve in said recirculation conduit, and a pneumatic servo connected to operate said exhaust-admission valve in response to movement of said pressure sensitive member and thereby maintain substantially atmospheric pressure at said location whenever said exhaust gas is recirculated, so that said metering orifice governs the rate of recirculation through said conduit as a substantially constant percentage of the mass airflow rate of the engine.

2. An internal combustion engine according to claim 1, in which the exhaust-admission valve is of disc-like form and is operated by a remote valve stem which is so located that it is removed from the influence of the recirculated exhaust gas.

3. An internal combustion engine according to claim 1, in which the exhaust-admission valve is rigidly connected to the remote valve stem by a hook-like yoke which operates within a chamber that leads from the inlet manifold at a location upstream of that at which the recirculated exhaust gas enters this manifold.

4. An internal combustion engine according to claim 1, in which the exhaust gas metering orifice is established by an annular seat receiving a reciprocatable plug-like valve which penetrates the orifice and which, whenever it moves, exerts on the surrounding edge of the orifice a rubbing action which ensures maintenance of the original efi'ective area of the orifice.

5. An internal combustion engine according to claim 1, in which the exhaust-admission valve is of circular form and is rigidly connected to one end of a hollow valve stem which passes, through a sliding pressure seal, into a casing containing three diaphragmchambers formed by two diaphragms of which one constitutes the pressure-sensitive member and the other constitutes the pneumatic servo; the hollow valve stem extending through the servo-diaphragm into the intermediate diaphragm-chamber, which is in permanent communication with atmosphere.

6. An internal combustion engine according to claim 5, in which the end of the hollow valve stem remote from the exhaust-admission valve has an orifice which is located within the intermediate diaphragm-chamber and which coacts with a disc-like valve fixed to the pressure-sensitive diaphragm; the valve stem has a second orifice placing its interior in permanent communication with the diaphragm-chamber formed by the servo-diaphragrn; and a restrictor is fitted in the valve stem either adjoining the exhaust-admission valve or at a iocation between that valve and the said second orifice.

7. An internal combustion engine according to claim 1, in which the servo-diaphragm is spring loaded in the direction to close the exhaust-admission valve, and the relation between the spring-load and the effective area of this diaphragm is such that the exhaust valve is opened when a depression slightly less than that prevailing when the engine is operating at full load acts on the servo-diaphragm.

8. An exhaust gas recirculation system as claimed in claim 1 which comprises a pressure-sensing valve assembly connected to respond to the depression obtaining at the location of a vacuum-advance drilling in a carburetter supplying said engine and to prevent the pressure-control valve assembly from permitting recirculation when said depression indicates that said engine is idling, overrunning, or operating at full load.

9. An exhaust gas recirculation system according to claim 8 comprising thermally responsive means connected to prevent said pressure-control valve assembly from permitting recirculation when the temperature of said engine is below about 60 F.

1: c a t UNITED STATES PATENT OFFICE CERTIFICATEOF CORRECTION I Peter Phillimore Swatman nventofls) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

The Assignee should read British Leyland (Austin-Morris) Limited, Birmingham, England, a corporation of Great Britain Signed and sealed this 5th day of November 1974.

(SEAL) Attest:

MCCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents USCOMM-DC BOSTQ PGD FORM PO-105O (1069) u s eovznmnn nmnmr, orncr 930 

1. An exhaust gas recirculation system for an internal combustion engine equipped with an inlet manifold and an exhaust system, said recirculation system comprising a metering orifice in said exhaust system opening into a recirculation conduit which leads to said inlet manifold, and a pressure-control valve assembly comprising a pressuresensitive member subjected to atmospheric pressure on one side and to the pressure in said conduit at a location immediately downstream of the exhaust gas metering orifice on its other side, an exhaust-admission valve in said recirculation conduit, and a pneumatic servo connected to operate said exhaustadmission valve in response to movement of said pressure sensitive member and thereby maintain substantially atmospheric pressure at said location whenever said exhaust gas is recirculated, so that said metering orifice governs the rate of recirculation through said conduit as a substantially constant percentage of the mass air-flow rate of the engine.
 2. An internal combustion engine according to claim 1, in which the exhaust-admission valve is of disc-like form and is operated by a remote valve stem which is so located that it is removed from the influence of the recirculated exhaust gas.
 3. An internal combustion engine according to claim 1, in which the exhaust-admission valve is rigidly connected to the remote valve stem by a hook-like yoke which operates within a chamber that leads from the inlet manifold at a location upstream of that at which the recirculated exhaust gas enters this manifold.
 4. An internal combustion engine according to claim 1, Pg,19 in which the exhaust gas metering orifice is established by an annular seat receiving a reciprocatable plug-like valve which penetrates the orifice and which, whenever it moves, exerts on the surrounding edge of the orifice a rubbing action which ensures maintenance of the original effective area of the orifice.
 5. An internal combustion engine according to claim 1, in which the exhaust-admission valve is of circular form and is rigidly connected to one end of a hollow valve stem which passes, through a sliding pressure seal, into a casing containing three diaphragm-chambers formed by two diaphragms of which one constitutes the pressure-sensitive member and the other constitutes the pneumatic servo; the hollow valve stem extending through the servo-diaphragm into the intermediate diaphragm-chamber, which is in permanent communication with atmosphere.
 6. An internal combustion engine according to claim 5, in which the end of the hollow valve stem remote from the exhaust-admission valve has an orifice which is located within the intermediate diaphragm-chamber and which coacts with a disc-like valve fixed to the pressure-sensitive diaphragm; the valve stem has a second orifice placing its interior in permanent communication with the diaphragm-chamber formed by the servo-diaphragm; and a restrictor is fitted in the valve stem either adjoining the exhaust-admission valve or at a location between that valve and the said second orifice.
 7. An internal combustion engine according to claim 1, in which the servo-diaphragm is spring loaded in the direction to close the exhaust-admission valve, and the relation between the spring-load and the effective area of this diaphragm is such that the exhaust valve is opened when a depression slightly less than that prevailing when the engine is operating at full load acts on the servo-diaphragm.
 8. An exhaust gas recirculation system as claimed in claim 1 which comprises a pressure-sensing valve assembly connected to respond to the depression obtaining at the location of a vacuum-advance drilling in a carburetter supplying said engine and to prevent the pressure-control valve assembly from permitting recirculation when said depression indicates that said engine is idling, overrunning, or operating at full load.
 9. An exhaust gas recirculation system according to claim 8 comprising thermally responsive means connected to prevent said pressure-control valve assembly from permitting recirculation when the temperature of said engine is below about 60* F. 